Means for and method of accelerating induction motors



lR. E. HEL'LMUND.

MEANS FOR AND METHOD 0F ACCELERATING INDUCTION MOORS.

APPLICATION FILED AUG-1D, |918. K v 1,417,731. 'Patented jmy 30,1922.

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, INVENTOR WITNESSES: I a( www. /ff/d/ff//e//mnd BY v 7 ATTRNEY i ,UNITED STATES PATENT OFFICE.

HOUSE ELECTRIC &; MANUFACTURING COMPANY, Ay CORIPORATION OF PENNSYL- VANIA.

MEANS FOR AND METHOD OF ACCELERATING INDUCTION MOTORS.

Application filed August 10, 1918. Serial No.

T 0 all 'wi/0m t 'may concern Be it hnownthat I, RUDoL v E. HELLMUND, a Subject of the German Empire, and a resident of Swissvale, in the county of Allegheny and State of Pennsylvania, have in` vented a new and useful Improvement in Means for i and Methods of Accelerating Induction Motors, of which the following is a specification.

My invention relates to means for, and methods of,-accelerating induction motors which are operated in conjunction with a variable primary frequency, especially if singlephase frequency changers are used.

In the operation of motors of the above character, in connection with combined single-phase frequency changers and phase converters, certain difficulties are experienced on account `of the higher harmonic secondary currents of such frequency changers. yThese currents will, when allowed to pass into the main machine, produce therein counter-torques. at certain speeds. These speeds may often be predetermined and I have discovered that, if, at these speeds, the ratio of voltage to frequency be raised an appreciable amount, these points of low torque may be passed without interfering with the acceleration of the motor or other harmful effects being noticed. However, unless the effects of the counter-torques are nullified, the large resultant currents at the critical speeds give undesirable heating and .low efficiency of operation for a given torque.

In determining the particular speeds at which the above-described counter-torques are most pronounced, use is made ofthe fact that only odd harmonic currents are effective in causing theundesirable fields. The singlephase magnetomotive forces which are im-` pressed upon a machine can be resolved into two oppositely rotating magnetomotive forces and the .fields produced in the stator members, rotate with the speed lof Rk'S and R+S, S being the synchronous speed of the rotor member of the machine and R being the actual speed of said member. `The currents `which are produced by these fields are commutated into' currents having a fres Specification of Letters Patent.

`current derived therefrom Patented May 30, 1922.

quency of fr-s and fr-l-s, fr being equal to the rotational frequency and f being equal to the line frequency.

y kIf the machine in question is operated at under synchronous speed, fr f and, there, fore, fr-f is negative. It follows that the algebraic difference of the two commutator frequencies is always Assuming now that one of the currents derived from the commutator has a frequency of f1, the other `will be equal to 2f f1n l nous speeds being figure.

If we now assume theycondition that the motor must run at approximately synchronous speed corresponding to the frequency 1, and, moreover, at approximately the syn` proportional to the latter chronous speed corresponding to of the frequency 2f-f1, we have f1 f1 T 2f f1 m 'where a may be any whole odd integer, since, ashereinbefore pointed out, it is well-known that only the odd harmonic currents set up undesirable fields. Assuming, for instance, that a=5, We have Lig and 2f-f1=q f,

The result is that a motor running at a speed corresponding to synchronism of one of the frequencies, at the same time, is running at all a speed corresponding toene-fifth of the' the resulting main motor synchro- CIB the ratio of voltage to frequency is increased while going through said speeds. Such an increase will result in a temporarily stronger main motor field and this will tend, in a large measure, to compensate for the loss in torque caused by the higher harmonic rurrents passing` into the main motor from its association freqiiency-changer secondary.

Referring to the drawings, in which a system for the practice of my method of operation is shown, Figure 1 is a circuit diagram illustrating the control used. Fi 2 is a graphic chart illustrating the manner in which the ratio of voltage Vto frequency is changed. Fig. 8 is a seq uence chart illustrating the switching operations necessary to obtain increased voltages in connection with the system shown in Fi g. l. Fig. l is a i'icw similar to Fig, l but illustrating` a modilication of the system, while Fig. 5 is a graphie ratio chart covering the operation of the system shown in Fig. Ll; and Figs. 6 and 'i are switching sequence charts illustrating the operation of the switches necessary to produce the ratios shown in Fig. 5.

Referring now more particularly to Vthe drawings, an induction motor 25, having a squirrel-cage secondary 26 and a phasewound primary Q7, is shown as supplied with energy through a frequcncy-changer 30, which consists of a rotor 3l and stator niembers 32m-32. The rotor of the `frequency changer is provided with slip rings 33 and a commutator 34, the slip rings being con'- nected to a source of supply, here shown,

for convenience, as the secondary 3G of a transformer 37. An auxiliary driving machine 85 is provided to operate the frequency changer 30. Switches l to 9, inclusive, are arranged in conjunction with .inductive devices 38 and 39 for the purpose of voltage control.

Referring now to Fig. 2, the manner in which it is necessary to increase the ratio of voltage to frequency is illustrated. The lines Gil-('35 and 66"-(37 indicate those frequencies at which it has been predetermined there an excess of higher harmonic currents passing into the main induction motor and in such manner as to cause points of low torque therein. In order, therefore, to adequately accelerate .the motor beyond tl point, it is necessary that, at such points, au increased ratio of voltage to frequency shall be obtained. For instance, in Fig. 2, it will be observed that the `line :S0-571 is that line at which each of the lines representing` frequency change would normally end. The il'requei'icy and voltage steps must therefore be so selected that none of the increases in frequency will fall tothe right of the lineEO-l, since, if such a condition were present, the motor would work with too weal': a field and might fail to accelerate beyond that speed, especially if an appreciable counter-torque should exist near this speed.

In the operation, therefore, of a system arranged in accordance with my invention, a switching sequence, as illustrated in Fig. 23, is used, and the voltage and frequency are raised in accordance with the chart alcove referred to and illustrated in Fig. 2. The frequency changer is first ope 'ated at its highest speed, that is, slightly below syn-i chronism, with the switches 3 and. 1- closed and its voltage adjusted for the lowest value by the closing of switches and 6. The steps illustrated by lines -52 and l32,-53 have then been taken and the main motor is operating at a voltage corresponding to the point 523. After the motor has been accelerated, under these conditions, as far as possible, and further acceleration is desired, the switches 2 and 3 are first closed to lower the speed of the frequency changer and thus to raise the frequency and the motor speed until point ff on the line :E0-5l is reached, this being a running point. To further accelerate the motor. the voltage is first raised by closing the switches 6 and 7. Switches l and 2 are then closed to lower the speed of the frequency changer and therfhiY graffiually increase the secondary i eqnency` It will be noted that. by this operation, the critical point, indicated by the line calf-G5, will he passed with a greatly increased ratio of voltage to frequency. The nent point of low torque, namely. that indicated by the line (3G-67, is passed `in a similar manner, as may be seen by reference to the sequence chart in Fig. 3.

In Fig. 4f, a system of acceleration is illustrated wherein the normal ratio `of voltage to frequency is indicated by a line which does not` pass through the intersection of the coordinates, that is, a line as illustrated at :5l-52.

This system gives, for eren normal operation, a somewhat increased ratio of voltage to frequency for the lower frequencies, such rmeration being desirable because ot the fact that the ohmic drops at these lower frequencies represent a higher percentage of the total motor voltage and also that, with these lower frequencies. a soinewliat stronger field .is both permissible and desirable to give minimum motor heating under all condi,- tions.

T he switching sequence used under these circumstances is shown in Fig. G and is similar to that .illustrated in Fig. 3.

However. .l have, in addition. illustrated in the graphie chart shown in Fi and in the sequence chart shown in Fig. 7, a method of even further increasing the abnormal ratio of voltage to frequency and. after thus passing the critical points. returning to normal ratio. In Fig, 5, 'these abnormal ratio increases are shown. in the dotted lines. By the use of this latter method of acceleralli.

tion, points of very decided counter-torque may be passed Without interfering with the normal operation of the motor. i

While I have shown but tvvo modifications of a system for accelerating an induction motor inaccordance with my invention, I do not desire to be limited to the specific means shovfuherein, since it is entirely possible that other combinations may be used for passing the hereinbefore described points of loiv torque Without departing from the scope of my invention. I desire, therefore, that the appended claims shall be construed. as broadly as the prior art will permit and be restricted only as hereinbefore pointed out or as their terms demand.

I claim as my invention:

l, The combination with a polyphase induction motor, and a combined frequency changer and phase-converter connected thereto, whereby relatively low-torque conditions are produced at certain critical speeds, of means for controlling the frequency of the current delivered to said induction motor, and means for so regulating the voltage impressed thereupon that the ratio of said voltage to said frequency is increased while said induction motor is passing through its critical speeds. i

2. The method of accelerating a polyphase induction motor from a combined frequency changer and phase-converter, which consists in independently,regulating the frequency of the current delivered to said induction motor and the voltage impressed Jthereupon in such manner that the ratio of said voltage to said frequency is increased While said induction motor is passing through its critical speeds.

3. In the combination of a polyphase in.- duction motor fed from a frequency changer connected to a single-phase source of supply, said motor having, by reason of said connection, points of low-torque or critical speed in its acceleration range, the method of accelerating said motor, Which consists in increasing the ratio of the applied voltage to frequency at such points of critical lowtorque speed.

4l. In the combination o'f a polyphase induction motor fed from a frequency `changer i connected to a single-phase source of supply, saidr motor having, by reason of said connec-tion, points of low-torque or critical speed in its acceleration range, the method of accelerating said motor, which consists in temporarily strengthening the field of said induction motor While it is passing through said low-torque points.

5. In the combination of a polyphase in-` duction motor fed from a frequency changer connected to a single-phase source of supply, said motor having, by reason of said connection, points of low-torque or critical speed in its acceleration range, the method of accelerating said motor, which consists in increasing the ratio of the applied voltage to frequency at such points of critical speed, and using said points ofr increased ratios as accelerating speeds only.

6.111 the combination of a polyphase induction motor fed from a frequency changer connected to a single-phase source of-supply, said motor having, by reason of said connection, points of low-torque or critical speed in its acceleration range, the method of step- `by-step acceleration of said motor, which consists in changing from one speed to another, in each acceleration-step, by first raising the voltage impressed on said motor, and thereafterincreasing the frequency of the current supplied thereto.

7. In the combination of a polyphase induction motor fed from a frequency changer connected to a single-phase source of supply, said motorhaving, by reason of said connection, points of low-torque or critical speed in its acceleration range, the method of accelerating said motor, Which comprises establishing, at starting, a ratio of impressed voltage on said motor to the frequency of current supplied thereto which is higher than the normal ratio of said quantities, and gradually decreasing the abnormality of said ratio during the progressive acceleration steps. v

8. In the combination of a polyphase induction motor fed from a frequency changer connected to a single-phase source of supply, said motor having, by reason of said con nection, points ofloW-torque or critical speed in its acceleration range, the method of accelerating said motor, Which comprises establishing, at starting, a ratio of impressed voltage on said motor to the frequency of current supplied thereto which is higher than the normal ratio of said quantities, and gradually decreasing the abnormality of said ratio during the progressive acceleration steps, and linally operating said motor with the normal ratio of said quantities.

9. In the combination of a polyphase induction motor fed from a frequency changer connected to a single-phase source of supply, said motor having, by reason of said connection, points of low-torque or critical speed in` its acceleration range, the method of accelerating said motor, `which comprises establishing, at starting, a ratio of impressed. voltage on said motor to thefrequency of current supplied thereto which is higher than the normal ratio of said quantities, gradually decreasing said ratio during the progressive acceleration steps and further increasing, at each critical speed, said ratio over that dictated by said gradually decreasingexcess.

10. The method of accelerating an induction motor from a source of `variable frequency and variable voltage Which consists the abnormality ofy in varying first *the voltage and then the 1`irfrequency or' said 'froquoncy changer fhu'rryf quoucy (hrrng ouch step of the acceleration each step oi' tho acceleration of Sad indou of smid mlucton motor. tion motor.

ll. The method oif accelerating un nducn tos'mony whereof, I have hereunto 5 tion motor associated wh L frequoucy Sllbsorbed my name this 31st day of July,

changer which consis'ss in taking stops tend- 1918. 111g to vary 'rst the voltage and thon tho RUDOLF E. HELLMUND, 

